The difficulties associated with the consistent electrodeposition of bright, conventional chromium deposits coupled with the imposition of government restrictions on the discharge of toxic effluents including hexavalent chromium present in conventional chromium electroplating baths has prompted the development of alternative electroplating bath compositions and techiques for depositing metal alloys intended to duplicate the color and characteristics of conventional chromium deposits. In U.S. Pat. No. 3,881,919, for exaple, an electroplating bath is disclosed for depositing a ternary alloy consisting of cobalt, tin and zinc which simulates a chromium deposit. In U.S. Pat. No. 4,035,249 which is assigned to the same assignee as the present invention, an electroplating bath composition is disclosed for depositing a binary alloy consisting of cobalt and tin. The bath composition and process as disclosed in the last mentioned U.S. patent is primarily adapted for the bulk plating of small workpieces such as in barrels and some difficulty has been encountered in adapting the bath for rack plating of workpieces.
While the various alloy electrodeposits suggested in accordance with such prior art patents have produced platings which simulate a conventional chromium electrodeposit, the resulting deposits and the process for their electrodeposition have had shortcomings detracting from a more widespread commercial acceptance thereof. For example, such alloy deposits have lacked the necessary corrosion resistance under moderate exposure conditions resulting in tarnish or color change. The hardness of such alloy deposits has also been substantially lower than that of a conventional chromium deposit. An increase in the corrosion resistance of such alloy deposits by the application of thicker electrodeposits has been limited due to the loss of chromium-like appearance necessitating the use of relatively thin electrodeposits in the order of about 0.02 to about 0.03 mils (0.00002 to about 0.00003 inch). Additionally, difficulties have been encountered in maintaining proper bath stability particularly in electrolytes containing stannous ions because of their tendency to become oxidized to the stannic state.
The bath composition and method of the present invention overcomes many of the problems associated with prior art compositions and methods for applying simulated chromium electrodeposits by providing a bath composition which is relatively easy to control, is stable, and is versatile in use for both rack and bulk plating processes. Additionally, the chromium-like deposit is possessed of increased hardness and corrosion resistance and can be deposited in thicknesses as high as 1 mil (about 25 micrometers) without encountering an adverse color change or a spongy physical structure. The alloy electrodeposit of the present invention can further be improved in its corrosion and tarnish resistance by the application of a passivating rinse following electrodeposition, such as a chromium rinse.